Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS4584548 A
Publication typeGrant
Application numberUS 06/722,782
Publication dateApr 22, 1986
Filing dateApr 12, 1985
Priority dateApr 26, 1984
Fee statusLapsed
Also published asDE3514818A1, DE3514818C2
Publication number06722782, 722782, US 4584548 A, US 4584548A, US-A-4584548, US4584548 A, US4584548A
InventorsYuji Inoue, Hideto Iwaoka, Tadashi Sugiyama
Original AssigneeYokogawa Hokushin Electric Corporation, Yokogawa Medical Systems, Limited
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Magnetic field generating coil assembly for nuclear magnetic resonance imaging apparatus
US 4584548 A
Abstract
A coil assembly comprises four coils for generating a static magnetic field in an NMR imaging apparatus, wherein the dimensions of the coils are selected so that the flatness of the cross section of the coils, the current turns ratio, and the radius ratio, are within certain limits. The assembly so constructed produces highly uniform magnetic field, is small in size, and light-weight, and consumes a lower amount of power, and can be manufactured inexpensively.
Images(6)
Previous page
Next page
Claims(2)
What is claimed is:
1. A coil assembly for generating a magnetic field comprising a pair of inner coils and a pair of outer coils, each of said coils having dimensions which satisfy the following ranges
a/b of the inner coil=0.4;
a/b of the outer coil=1.0;
0.6=rc =1.0; and
0.8=R1 Rs=1.2;
wherein a/b is ratio of cross sectional flatness, rc is ratio of product of current in the outer coil and number of turns thereof with respect to product of current in the inner coil and number of turns thereof; and R1 /Rs is ratio of radius of the inner coil with respect to radius of the outer coil; and wherein a Taylor expansion of an axial magnetic field at the center of said inner coils and outer coils has eliminated second, fourth and sixth terms.
2. The assembly of claim 1, wherein said coils have the dimensions, depth 1200 mm, width 1100 mm, clear bore diameter 800 mm, weight 1900 Kg, and consumed power 47.0 Kw, at 1500 Gauss magnetic field intensity.
Description
BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a magnetic field generating coil assembly for use in a nuclear magnetic resonance imaging apparatus.

2. Description of the Prior Art

Nuclear magnetic resonance (hereinafter referred to as "NMR") is effective in providing a microscopic understanding of substances in the fields of solid state physics and chemistry.

The NMR resides in a spectroscopic means which weakly coacts with a measured system through the use of a uniform static magnetic field and a weak RF magnetic field. The energy of the RF magnetic field normally used by the NMR ranges from 10-19 to 10-20 erg, which is substantially weaker than X-ray energy which is used in the range of from 10-8 to 10-10 erg.

These features of the NMR are of interest in the medical field since they provide an undestructive biological measuring technique. Particularly, its clinical application as an NMR-CT (computerized tomography) has been studied under the stimulus of the report by R. Damadian, Science Vol. 171, Page 1151 (1971), which discloses that the nuclear magnetic relaxation time of the hydrogen atomic nuclei of water molecules of a malignant tumor is several times longer than that of a normal tumor.

That NMR phenomenon is utilized in an NMR imaging apparatus for producing the image of a sectional plane of a subject to be examined. The NMR imaging apparatus ordinarily uses, as a static field generating coil, a normal conductivity magnetic assembly, such as shown in FIG. 1, for the reason that the illustrated magnet assembly performs well in examining human bodies.

For producing good images, the magnetic field uniformity in an imaging region should be of about 10 ppm. To meet this requirement, various designing methods have been heretofore proposed. The publication "NMR medicine" issued on Jan. 20, 1984 by the Society for the Research of NMR Medicine describes, on pages 78 and 89, the following designing method.

With the four coils disposed symmetrically as shown in FIG. 1, it is possible to eliminate all of the second, fourth and sixth term errors of the magnetic field. These coils are called double Helmholz coils, which are widely used in a normal conductivity NMR imaging magnet.

The conventional conditions for designing the foregoing coil system are as follows (see FIG. 1 for definitions of the symbols):

(A) Where the coils can approximate a concentrated current loop, the spacing between the coils are selected to be as follows:

cos θ1 =z1 /R0 =0.76506

cos θ2 =z2 /R0 =0.28523

The ratio of ampere turns of the coils is

AT2 /AT1 =1.4660

At this time, the second, fourth, and sixth term errors of the magnetic field are all eliminated, providing an eighth compensating coil.

(B) With respect to double Helmholz coils having rectangular cross sections of finite dimensions, the following dimensions are selected:

cos θ1 =z1 /R0 =0.76506

cos θ2 =z2 /R0 =0.28523

The ratio of ampere turns of the coils is

AT2 /AT1 =1.46608

If the current densities in the respective coils are equal, then

a1 /a2 =0.67188

b1 /b2 =1.01523

The dimensions of coils, a1, a2, b1, b2 are selected to meet the relationship:

a1, a2, b1, b2 <<R0.

With the dimensions selected as above, the second and fourth term errors of the magnetic field are eliminated, providing a sixth compensating coil.

(C) With respect to double Helmholz coils having rectangular cross sections of finite dimensions, the dimensions for providing an eighth compensating coil are selected according to the below Table 1.

In the coil system thus designed, the magnetic field produced thereby is expressed by

Bz(z,0)=B0 (1+γs(z/R0)+ . . . )

In any case, the inner coils are vertically elongate with dimensions a2 /b2 ≧1. With these coils, the dimension (the maximum outer coil dimension of 1,600 mm), the power consumption (about 60 KW), and the weight (about 2000 Kg) are large, and the clear coil diameter C (defined by the diameter of the smaller coils) is small (about 700 mm).

              TABLE 1______________________________________Example 1    Example 2  Exaple 3  Example 4______________________________________b1 /2RO  0.04206   0.09386    0.07503 0.08116z1 /RO  0.76598   0.76847    0.76346 0.76015z2 /RO  0.28501   0.28519    0.28638 0.28700Nb1  32        20         15      15Na1  16        15         20      24Nb2  25        20         21      23Na2  30        22         21      23γ8  -2.106    -2.101     -2.083  -2.080______________________________________
SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a coil assembly for generating a highly uniform static magnetic field, which coil assembly is small in size, light in weight, consumes a a reduced amount of electric power, and is inexpensive to manufacture.

According to the invention, there is provided a coil assembly for generating a magnetic field, comprising a pair of inner coils and a pair of outer coils with dimensions selected to meet the following conditions:

a/b of the inner coil=a/b≦0.4

a/b of the outer coil=a/b≦1.0

0.6≦rc ≦1.0

0.8≦R1 /Rs≦1.2

wherein a/b is the cross sectional flatness; rc is the ratio of the product of a current in the outer coil and the number of turns thereof with respect to the product of a current in the inner coil and the number of turns thereof; and R1 /Rs is the ratio of the radius of the outer coil about the center of the cross section thereof with respect to the radius of the inner coil about the center of the cross section thereof; and wherein the Taylor expansion of an axial magnetic field at the center of the coils having eliminated second, fourth and sixth terms thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view of a conventional static magnetic field generating coil assembly.

FIGS. 2 and 3 are diagrams which explain the relationship between loop current and magnetic field.

FIGS. 4 through 9 are diagrams dipicting the relationship between coil shapes and characteristics.

FIG. 10 is a view of a coil assembly according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Principles of the Invention

(I) Based on the Biot-Savart law, the component in the z direction of the magnetic field H(z) produced on the z axis by a loop current, as shown in FIG. 2, is expressed as follows: ##EQU1## Hence

H(z)=∫d =IR2 /2(R2 +Z2)3/2        (1)

wherein

R=the radius of the loop

s=the length along the loop.

H=the vector of the magnetic field.

φ=the angle between ds and the Z axis on an arbitrary point on the Z axis.

r=the distance from the arbitrary point on the X axis to ds.

I=the current.

(II) As shown in FIG. 3, two loop currents are positioned parallel to each other at the spacing of 2d. With the two coils having their center at z=0, the magnetic field H1, H2 produced by the respective coils are given by

H1 (z)=H(z-d)                                         (2)

H2 (z)=H(z+d)                                         (3)

By subjecting equations (2) and (3) to Taylor expansion with z=0, and expressing them by using H(z), they are converted into the following equations (4) and (5), respectively. ##EQU2##

Since the magnetic field produced by the two loop currents is expressed by H1 +H2, equations (4) and (5) are added together to obtain ##EQU3## Therefore, the terms of z to the odd numbered powers are eliminated.

Thus, where the coils are disposed symmetrically with z=0 only the terms of z to the even numbered powers remain in the Taylor expansion at z=0.

The coefficients are specifically calculated from the equation (1) as indicated by equations (7), (8) and (9). For easier understanding, x=z/R and the term of 1/2 and 1/n! are omitted. ##EQU4##

(III) While the linear currents have been described, four coils having finite cross sectional areas can be analyzed in the same manner. As with the case of the two coils, only the terms of z to the even numbered powers appear in the Taylor expansion of the magnetic field at z=0. The coefficients can be obtained by adding (integrating) linear currents which are regarded as being distributed in a cross sectional plane.

(IV) The Taylor expansion of the magnetic field at z=0 is obtained in the manner described above. A magnetic field h(z) produced on the z axis by four coils having finite cross sectional areas can be expanded according to equation (10) at z=0.

h(z)=a0 +a2 z2 +a4 z4 +a6 z6 + . . . (10)

If |z|<1, the

|z|n <|z|m   (n<m)

Therefore, those coefficients at lower degrees are more responsible for affecting the uniformity of the magnetic field.

By determining the shape so that the three terms of |a2 |, |a4 |, |a6 | (or (|a2 |2 +|a4 |2 +|a6 |2)1/2 will be minimized, the coils can produce a static magnetic field which is highly uniform.

REFERENCES FOR EVALUATING A STATIC MAGNETIC FIELD

(i) The following four functions are generally used for evaluating magnets.

(1) The uniformity and region of a magnetic field.

(2) The weight of a coil.

(3) The amount of consumed power.

(4) The clear bore diameter.

If the coil weight is reduced, then the consumed power is increased. If the clear bore diameter is increased, then the coil weight and the consumed power become larger. Since the above four evaluating factors are not directly proportional to each other, and some contradict another, it is not simple to evaluate the performance of a megnet from more consideration of data obtained from these four factors (i)(1) through (i)(4).

With the static magnetic field magnet, however, uniformity is required and the magnetic field intensity is limited. Accordingly, the above data items (i)(2), (i)(3) and (i)(4) should be compared under conditions which produce the same magnetic field intensity in the same uniformity region (for example, in which the uniformity is 10 ppm or less).

(ii) As a consequence, the following new evaluating factor or function has been introduced.

Within the range in which the coil shape is not largely changed, the product of the coil weight and the consumed power remains constant.

Consumed power=P=I2 (2πR/S)ρN

Weight=W=S.2πRNδ

Hence, ##EQU5## wherein I=the current

S=the cross sectional area of a single wire.

ρ=the resistivity of the wire.

N=the number of turns of wire.

δ=the weight of the wire per unit volume.

R=the diameter of a coil.

Accordingly, the smaller the product of the consumed power and the coil weight, and the larger the clear bore diameter, the better the static magnetic field coil.

The formula or factor (consumed power multiplied by the coil weight=PW) is therefore used as an important evaluating function.

RELATIONSHIP BETWEEN COIL SHAPES AND CHARACTERISTICS

(a) Where the cross sectional area of a coil is relatively small as compared to the overall coil. The four coils are divided into inner and outer coils. Similar to the configuration of FIG. 1, the two inner coils are identical in shape, and so are the two outer coils. The inner and outer coils have different numbers of wire turns. Equal currents are passed through the wires of the inner and outer coils.

FIG. 4 shows how the factor (weight multiplied by consumed power) varied with respect to the ratio (hereinafter referred to as γ) of the turns of the inner coil to the turns of the outer coil. As is apparent from FIG. 4, the factor (weight multiplied by consumed power) is minimized in the vicinity of γ=1.0 through 1.5.

FIGS. 5(a) and 5(b) show different coils shapes with respect to the ratio.

(b) Where the cross sectional area of the coil is so large as compared with the entire coil, that it is not negligible, the cross sectional shape affects the characteristics of the coil as the magnet. Most of the actual coils used fall under this category.

FIG. 6 shows the manner in which the factor (weight multiplied by the consumed power) is varied when the cross sectional shape of the coil is changed from a horizontal elongate shape to a vertically elongate shape. FIG. 6 indicates that the horizontally elongate shape is better than the vertically elongate shape for the reason that the diameter of the coil with the horizontally elongate cross section is smaller as a whole. The difference between the shapes of the outer coils does not essentially affect the characteristics.

(c) FIGS. 7 and 8 show a combination of the varying cross sectional shape of the coil as described above in item (b) and the varying γ as described above in item (a). FIG. 7 indicates how the diameters of the inner and outer coils vary with respect to γ. At γ=0.6 through 1.0, the diameters of the outer and inner coils are substantially equal to each other and the clear bore diameter is at maximum.

FIG. 8 indicates how the factor (weight multiplied by consumed power) varies with respect to γ. As with FIG. 7, the factor (weight multiplied by consumed power) is maximum at γ=0.6 through 1.2. In FIGS. 7 and 8, the inner coil has a horizontally elongate cross section.

FIG. 9 shows the relationship between the clear bore diameter and the factor (weight multiplied by consumed power). That factor is progressively lowered along the solid line curves in FIG. 9. If the cross sectional shape of the inner coils were too horizontally elongate, the inner coils would interfere with each other. The dotted line curved in FIG. 9 represents a limitation on the horizontal elongation of the cross section of the inner coils. As can be understood from FIG. 9, when a certain clear bore diameter is selected, which minimizes the factor (weight multiplied by consumed power) is uniquely determined. The inner coils should be horizontally elongate to an extent just short of mutual contact.

FIG. 10 shows in cross section, a coil assembly according to the invention. Indicated by dotted lines is a conventional coil assembly. Table 2 shows characteristic dimensions and properties of an illustrative embodiment of the invention as compared with a conventional coil assembly. The coil assembly having the dimensions of Table 2 provide a magnet which can produce a uniform magnetic field in a large region, is small in size, light in weight, and consumes a lower amount of electric power than the conventional assemblies. Other dimensions may be selected within the ranges set forth below to produce magnet assemblies which have similar advantageous properties.

The inventors have discovered that coils having the following ranges would produce the aforementioned and other advantages.

a/b of the inner coil=a/b≦0.4.

a/b of the outer coil=a/b≦1.0.

0.6≦rc ≦1.0.

0.8≦R1 /Rs≦1.2

wherein

a/b=the flatness ratio of the coil cross section.

rc =the ratio of the current x number of turns in the inner coil to the current x number of turns in the outer coil.

R1 /Rs=the ratio of the radius of the inner coil to the radius of the outer coil.

The coil assembly will satisfy the condition wherein the Taylor expansion of an axial magnetic field at the center of the coil has eliminated the second, fourth and sixth terms.

The coil assembly of FIG. 10 has dimensions which conform to the above ranges, and in one illustrative embodiment have dimensions shown in below Table 2.

              TABLE 2______________________________________                     Invention           Prior Coil                     Coil______________________________________Depth             1133   mm       1200 mmWidth (height)    1608   mm       1100 mmClear Bore diameter             700    mm .0.   800  mm .0.Weight            2000   Kg       1900 KgConsumed Power (40 C.)             60.0   KW       47.0 KWUniform regionDiametrical       350    mm       350  mmz direction       100    mm       300  mmMagnetic Fieldintensity         1500   Gauss    1500 Gauss______________________________________

With the arrangement of the invention, a normal conductivity coil assembly comprising four coils for generating a static magnetic field is advantageously small in size, lightweight, and consumes a low amount of electric power. The inner and outer coils are constructed to be of selected dimensions so that the flatness ratio of the cross section, the current ratio, and the coil radius ratio, meet the above prescribed ranges.

The foregoing description is illustrative of the principles of the invention. Numerous modifications and extensions thereof would be apparent to the worker skilled in the art. All such modifications and extensions are to be considered to be within the spirit and scope of the invention.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US4320342 *Sep 14, 1979Mar 16, 1982U.S. Philips CorporationMagnet coil arrangement for generating linear magnetic gradient fields
US4362993 *Aug 5, 1980Dec 7, 1982Picker International LimitedImaging systems
US4398150 *Feb 2, 1981Aug 9, 1983Thomson - CsfIronless high-homogenety magnet and its application to nuclear magnetic resonance imaging
US4509011 *Apr 30, 1982Apr 2, 1985Tokyo Shubaura Denki Kabushiki KaishaNuclear magnetic resonance diagnostic apparatus
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4660013 *Sep 23, 1985Apr 21, 1987General Electric CompanyCompact whole body resistive magnetic resonance imaging magnet
US4710741 *Dec 3, 1986Dec 1, 1987Picker International Ltd.Electromagnet arrangements for producing a magnetic field of high homogenity
US4716370 *Sep 20, 1985Dec 29, 1987Yokogawa Electric CorporationCoil arrangement for producing static magnetic field
US4799017 *Mar 12, 1987Jan 17, 1989Siemens AktiengesellschaftBackground field magnet for image generating devices using nuclear spin resonance
US5113166 *Dec 8, 1989May 12, 1992Hitachi, Ltd.Magnetic field gradient coil system
US6822451Jul 31, 2002Nov 23, 2004Ge Medical Systems Global Technology Company LlcNon-coupling magnetic sheilding coil
US7064548 *Apr 30, 2004Jun 20, 2006The Boc Group, Inc.RF probe apparatus for NMR check weighing system
US7199581 *Apr 30, 2004Apr 3, 2007The Boc Group, Inc.Magnetic resonance measuring system determining the mass of samples in a production line with monitored drift compensation
US8648678Jul 8, 2011Feb 11, 2014Commissariat A L'energie Atomique Et Aux Energies AlternativesCompact superconducting magnet device
US9234949Jul 8, 2011Jan 12, 2016Commissariat A L'energie Atomique Et Aux Energies AlternativesSuperconducting magnet device
US20050242813 *Apr 30, 2004Nov 3, 2005The Boc Group Inc.RF probe apparatus for NMR check weighing system
WO2012007679A1 *Jul 8, 2011Jan 19, 2012Commissariat A L'energie Atomique Et Aux Energies AlternativesCompact superconducting magnet device
WO2012007680A1 *Jul 8, 2011Jan 19, 2012Commissariat A L'energie Atomique Et Aux Energies AlternativesSuperconducting magnet device
Classifications
U.S. Classification335/299, 324/320
International ClassificationH01F7/20, G01R33/3875, A61B5/055, G01R33/20, G01R33/381, A61B10/00
Cooperative ClassificationH01F7/20, G01R33/3875
European ClassificationH01F7/20, G01R33/3875
Legal Events
DateCodeEventDescription
Apr 12, 1985ASAssignment
Owner name: YOKAGAWA MEDICAL SYSTEMS, LIMITED, 7-11, NAKACHO,
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNORS:INOUE, YUJI;IWAOKA, HIDETO;SUGIYAMA, TADASHI;REEL/FRAME:004397/0560
Effective date: 19850402
Owner name: YOKOGAWA HOKUSHIN ELECTRIC CORPORATION, 9-32, NAKA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNORS:INOUE, YUJI;IWAOKA, HIDETO;SUGIYAMA, TADASHI;REEL/FRAME:004397/0560
Effective date: 19850402
Feb 4, 1986ASAssignment
Owner name: YOKOGAWA MEDICAL SYSTEMS LIMITED 1-3 SAKAECHO, 6-C
Free format text: EXTRACT FROM CORPORATE REGISTER, TOKYO SHOWING CHANTE OF ASSRESS EFFECTIVE 12/07/85;ASSIGNOR:YOKOGAWA MEDICAL SYSTEMS, LIMITED, 7-11, NAKACHO 2-CHOME, MUSASHINO-SHI, TOKYO, JAPAN, A CORP. OF JAPAN;REEL/FRAME:004512/0593
May 26, 1987ASAssignment
Owner name: YOKOGAWA ELECTRIC CORPORATION
Free format text: CHANGE OF NAME;ASSIGNOR:YOKOGAWA HOKUSHIN ELECTRIC CORPORATION;REEL/FRAME:004748/0294
Effective date: 19870511
Jun 5, 1989FPAYFee payment
Year of fee payment: 4
Aug 17, 1993FPAYFee payment
Year of fee payment: 8
Feb 13, 1998REMIMaintenance fee reminder mailed
Apr 19, 1998LAPSLapse for failure to pay maintenance fees
Jun 30, 1998FPExpired due to failure to pay maintenance fee
Effective date: 19980422